1. Field of the Invention
This invention relates to an improvement of a fundus camera which takes fluorescent photographs by infra-red light and by light other than infra-red light.
2. Description of the Prior Art
Conventionally, a fundus camera is known in the prior art which is capable of taking color photographs or black and white photographs of the fundus of a subject's eye by illuminating it with visible light, and which is also capable of taking fluorescent photographs of the fundus by illuminating it with visible light thereby exciting a fluorescent agent to fluoresce.
In this type of fundus camera, the amount of light from the photographic light source was automatically set depending on the photographic mode employed. Further, provision was also made for changing the amount of light emitted by the source by means of a manual switch.
However, fundus cameras are now being developed which are capable of taking fluorescent photographs by infra-red light in addition to color photographs by visible light and fluorescent photographs by visible light.
To use such an infra-red fundus camera, the fluorescent agent indocyanin green is first injected into a vein of the subject. Indocyanin green has the property of combining with protein in the subject's blood. Under illumination by infra-red light, the indocyanin green combined with protein fluoresces. A photograph of the fundus can thus be obtained using this fluorescence excited by the infra-red illumination.
In this fluorescent photography by infra-red illumination, the exposure conditions of the subject's eye vary with the time elapsed after the injection. The operator must therefore manually adjust the amount of light emitted from the photographic light source depending on this variation, and it is not possible to automatically set the amount of light from the illuminating source beforehand as can be done with color photography by visible light or fluorescent photography by visible light.
There are also individual differences in the protein concentrations of subjects which lead to fluctuations in the amount of indocyanin green combined with protein. Due to the large number of unknown factors, therefore, it is difficult to set the amount of light emitted by the illuminating light source automatically.
One way of overcoming this problem would be to provide this type of fundus camera with a selector switch in the manufacturing stage which selects between automatic setting and manual setting of the amount of light emitted by the photographic light source.
If such an automatic/manual selector switch were provided, however, there is a risk that the operator might forget to operate it.
For example, the switch might be left in the automatic position when a good photograph could have been obtained in the fluorescent mode using infra-red light only with a manual setting. Conversely, the switch might be left in the manual position when a good photograph could have been obtained with an automatic setting in a mode other than the fluorescent mode using infra-red light (i.e. color photography with visible light, or fluorescent photography with visible light).
Alternatively, the camera could be set manually for all photographic modes. If automatic setting of the amount of light gives good photographs in the color mode using visible light and fluorescent mode using visible light, however, there is no need to go to the trouble of adjusting the camera manually, and although a selector switch would then be rendered unnecessary, the operation of the camera would be unnecessarily complicated.
The problem of adjusting the amount of light when taking photographs could not therefore be conveniently resolved whatever arrangement was adopted.